1. Field of the Invention
The present invention relates generally to an information transmitting apparatus and method in an IMT 2000 system, and in particular, to an apparatus and method for transmitting a transport format combination indicator (TFCI).
2. Description of the Related Art
A CDMA mobile communication system (hereinafter, referred to as an IMT 2000 system) generally transmits frames that provide a voice service, an image service, a character service on a physical channel such as a dedicated physical data channel (DPDCH) at a fixed or variable data rate. In the case where the data frames which include that sort of services are transmitted at a fixed data rate, there is no need to inform a receiver of the spreading rate of each data frame. On the other hand, if the data frames are transmitted at a variable data rate, which implies that each data frame has a different data rate, a transmitter should inform the receiver of the spreading rate of each data frame determined by its data rate. A data rate is proportional to a data transmission rate and the data transmission rate is inversely proportional to a spreading rate in a general IMT 2000 system.
For transmission of data frames at a variable data rate, a TFCI field of a DPCCH informs a receiver of the data rate of the current service frame. The TFCI field includes a TFCI indicating a lot of information including the data rate of a service frame. The TFCI is information that helps a voice or data service to reliably be provided.
FIGS. 1A to 1D illustrate examples of applications of a TFCI. FIG. 1A illustrates application of the TFCI to an uplink DPDCH and an uplink dedicated physical control channel (DPCCH). FIG. 1B illustrates application of the TFCI to a random access channel (RACH). FIG. 1C illustrates application of the TFCI to a downlink DPDCH and a downlink DPCCH. FIG. 1D illustrates application of the TFCI to a secondary common control physical channel (SCCPCH).
Referring to FIGS. 1A to 1D, one frame is comprised of 16 slots and each slot has a TFCI field. Thus, one frame includes 16 TFCI fields. A TFCI field includes NTFCI bits and a TFCI generally has 32 bits in a frame. To transmit the 32-bit TFCI in one frame, 2 TFCI bits can be assigned to each of the 16 slots (Tslot=0.625 ms).
FIG. 2 is a block diagram of a base station transmitter in a general IMT 2000 system.
Referring to FIG. 2, multipliers 211, 231, and 232 multiply input signals by gain coefficients G1, G3, and G5. Multipliers 221, 241, and 242 multiply TFCI codewords (TFCI code symbols) received from corresponding TFCI encoders by gain coefficients G2, G4, and G6. The gain coefficients G1 to G6 may have different values according to service types or handover situations. The input signals include pilots and power control signals (TPCs) of a DPCCH and a DPDCH data. A multiplexer 212 inserts 32 bit TFCI code symbols(TFCI codeword) received from the multiplier 221 into the TFCI fields as shown in FIG 1C. A multiplexer 242 inserts 32-bit TFCI code symbols received from the multiplier 241 into the TFCI fields. A multiplexer 252 inserts 32-bit TFCI code symbols received from the multiplier 242 into the TFCI fields. Insertion of TFCI code symbols into TFCI fields is shown in FIGS. 1A to 1D. The 32 code symbols are obtained by encoding TFCI bits(information bits) that define the data rate of a data signal on a corresponding data channel. 1st, 2nd, and 3rd serial to parallel converters (S/Ps) 213, 233, and 234 separate the outputs of the multiplexers 212, 242, and 252 into I channels and Q channels. Multipliers 214, 222, and 235 to 238 multiply the outputs of the S/Ps 213, 233, and 234 by channelization codes Cch1, Cch2, and Cch3. The channelization codes are orthogonal codes. A first summer 215 sums the outputs of the multipliers 214, 235, and 237 and generates an I channel signal and a second summer 223 sums the outputs of the multipliers 222, 236, and 238 and generates a Q channel signal. A phase shifter 224 shifts the phase of the Q channel signal received from the second summer 223 by 90°. A summer 216 adds the outputs of the first summer 215 and the phase shifter 224 and generates a complex signal I+jQ. A multiplier 217 scrambles the complex signal with a complex PN sequence Cscramb assigned to the base station. A signal processor(S/P) 218 separates the scrambled signal into an I channel and a Q channel. Low-pass filters (LPFs) 219 and 225 limits the bandwidths of the I channel and Q channel signals received from the S/P 218 by low-pass-filtering. Multipliers 220 and 226 multiply the outputs of the LPFs 219 and 225 by carriers cos(2πfct) and sin(2πfct), respectively, thereby transforming the outputs of the LPFs 219 and 225 to an RF (Radio Frequency) band. A summer 227 sums the RFI channel and Q channel signals.
FIG. 3 is a block diagram of a mobile station transmitter in the general IMT 2000 system.
Referring to FIG. 3, multipliers 311, 321, and 323 multiply corresponding signals by channelization codes Cch1, Cch2, and Cch3. Signals 1, 2, 3 are first, second and third DPDCH signal. An input signal 4 includes pilots and TPCs of a DPCCH.TFCI information bits are encoded into 32 bit TFCI code symbols by a TFCI encoder 309. A multiplier 310 inserts a 32 bit TFCI code symbols into the signal 4 as shown in FIG. 1A. A multiplier 325 multiplies a DPCCH signal which include TFCI code symbol received from the multiplier 310 by a channelization code Cch4. The channelization codes Cch1 to Cch4 are orthogonal codes. The 32 TFCI code symbols are obtained by encoding TFCI information bits that define the data rate of the DPDCH signals. Multipliers 312, 322, 324, and 326 multiply the outputs of the multipliers 311, 321, 323, and 325 by gain coefficients G1 to G4, respectably. The gain coefficients G1 to G4 may have different values. A first summer 313 generates an I channel signal by adding the outputs of the multipliers 312 and 322. A second summer 327 generates a Q channel signal by adding the outputs of the multipliers 324 and 326. A phase shifter 328 shifts the phase of the Q channel signal received from the second summer 327 by 90°. A summer 314 adds the outputs of the first summer 313 and the phase shifter 328 and generates a complex signal I+jQ. A multiplier 315 scrambles the complex signal with a PN sequence Cscramb assigned to a base station. An S/P 329 divides the scrambled signal into an I channel and a Q channel. LPFs 316 and 330 low-pass-filter the I channel and Q channel signals received from the SIP 329 and generate signals with limited bandwidths. Multipliers 317 and 331 multiply the outputs of the LPFs 316 and 330 by carriers cos(2πfct) and sin(2πfct), respectively, thereby transforming the outputs of the LPFs 316 and 330 to an RF band. A summer 318 sums the RF I channel and Q channel signals.
TFCIs are categorized into a basic TFCI and an extended TFCI. The basic TFCI represents 1 to 64 different information including the data rates of corresponding data channels using 6 TFCI information bits, whereas the extended TFCI represents 1 to 128, 1 to 256, 1 to 512, or 1 to 1024 different information using 7, 8, 9 or 10 TFCI information bits. The extended TFCI has been suggested to satisfy the requirement of the IMT 2000 system for more various services. TFCI bits are essential for a receiver to receive data frames received from a transmitter. That is the reason why unreliable transmission of the TFCI information bits due to transmission errors lead to wrong interpretation of the frames in the receiver. Therefore, the transmitter encodes the TFCI bits with an error correcting code prior to transmission so that the receiver can correct possibly generated errors in the TFCI.
FIG. 4A conceptionally illustrates a basic TFCI bits encoding structure in a conventional IMT 2000 system and FIG. 4B is an exemplary encoding table applied to a biorthogonal encoder shown in FIG. 4A. As stated above, the basic TFCI has 6 TFCI bits (hereinafter, referred to as basic TFCI bits) that indicate 1 to 64 different information.
Referring to FIGS. 4A and 4B, a biorthogonal encoder 402 receives basic TFCI bits and outputs 32 coded symbols(TFCI codeword or TFCI code symbol). The basic TFCI is basically expressed in 6 bits. Therefore, in the case where a basic TFCI bits of less than 6 bits are applied to the biorthogonal encoder 402, 0s are added to the left end, i.e., MSB (Most Significant Bit) of the basic TFCI bits to increase the number of the basic TFCI bits to 6. The biorthogonal encoder 402 has a predetermined encoding table as shown in FIG. 4B to output 32 coded symbols for the input of the 6 basic TFCI bits. As shown in FIG. 4B, the encoding table lists 32(32-symbol) orthogonal codewords c32.1 to c32.32 and 32 biorthogonal codewords {overscore (C32.1)} to {overscore (c32.32)} that are the complements of the codewords c32.1 to c32.32. If the LSB (Least Significant Bit) of the basic TFCI is 1, the biorthogonal encoder 402 selects out of the 32 biorthogonal codewords. If the LSB is 0, the biorthogonal encoder 402 selects out of the 32 orthogonal codewords. One of the selected orthogonal codewords or biorthogonal codewords is then selected based on the other TFCI bits.
A TFCI codeword should have powerful error correction capability as stated before. The error correction capability of binary linear codes depends on the minimum distance (dmin) between the binary linear codes. A minimum distance for optimal binary linear codes is described in “An Updated Table of Minimum-Distance Bounds for Binary Linear Codes”, A. E. Brouwer and Tom Verhoeff, IEEE Transactions on Information Theory, vol. 39, No. 2, March 1993 (hereinafter, referred to as reference 1).
Reference 1 gives 16 as a minimum distance for binary linear codes by which 32 bits are output for the input of 6 bits. TFCI codewords output from the biorthogonal encoder 402 has a minimum distance of 16, which implies that the TFCI codewords are optimal codes.
FIG. 5A conceptionally illustrates an extended TFCI bits encoding structure in the conventional IMT 2000 system, FIG. 5B is an exemplary algorithm of distributing TFCI bits in a controller shown in FIG. 5A, and FIG. 5C illustrates an exemplary encoding table applied to biorthogonal encoders shown in FIG. 5A. An extended TFCI is also defined by the number of TFCI bits. That is, the extended TFCI includes 7, 8, 9 or 10 TFCI bits (hereinafter, referred to as extended TFCI bits) that represent 1 to 128, 1 to 256, 1 to 512, or 1 to 1024 different information, as stated before.
Referring to FIGS. 5A, 5B, and 5C, a controller 500 divides TFCI bits into two halves. For example, for the input of 10 extended TFCI bits, the controller 500 outputs the first half of the extended TFCI as first TFCI bits (word 1) and the last half as second TFCI bits (word 2). The extended TFCI are basically expressed in 10 bits. Therefore, in the case where an extended TFCI bits of less than 10 bits are input, the controller 500 adds 0s to the MSB of the extended TFCI bits to represent the extended TFCI in 10 bits. Then, the controller 500 divides the 10 extended TFCI bits into word 1 and word 2. Word 1 and word 2 are fed to biorthogonal encoders 502 and 504, respectively. A method of separating the extended TFCI bits a1 to a10 into word 1 and word 2 is illustrated in FIG. 5B.
The biorthogonal encoder 502 generates a first TFCI codeword having 16 symbols by encoding word 1 received from the controller 500. The biorthogonal encoder 504 generates a second TFCI codeword having 16 symbols by encoding word 2 received from the controller 500.
The biorthogonal encoders 502 and 504 have predetermined encoding tables to output the 16-symbol TFCI codewords for the two 5-bit TFCI inputs (word 1 and word 2). An exemplary encoding table is illustrated in FIG. 5C. As shown in FIG. 5C, the encoding table lists 16 orthogonal codewords of length 16 bits c16.1 to c16.16 and biorthogonal codewords {overscore (c16.1)}, to {overscore (c16.16)} that are the complements of the 16 orthogonal codewords. If the LSB of 5 TFCI bits is 1, a biorthogonal encoder (502 or 504) selects the 16 biorthogonal codewords. If the LSB is 0, the biorthogonal encoder selects the 16 orthogonal codewords. Then, the biorthogonal encoder selects one of the selected orthogonal codewords or biorthogonal codewords based on the other TFCI bits and outputs the selected codeword as the first or second TFCI codeword.
A multiplexer 510 multiplexes the first and second TFCI codewords to a final 32-symbol TFCI codeword.
Upon receipt of the 32-symbol TFCI codeword, a receiver decodes the TFCI codeword separately in halves (word 1 and word 2) and obtains 10 TFCI bits by combining the two decoded 5-bit TFCI halves. In this situation, a possible error even in one of the decoded 5-bit TFCI output during decoding leads to an error over the 10 TFCI bits.
An extended TFCI codeword also should have a powerful error correction capability. To do so, the extended TFCI codeword should have the minimum distance as suggested in reference 1.
In consideration of the number 10 of extended TFCI bits and the number 32 of the symbols of a TFCI codeword, reference 1 gives 12 as a minimum distance for an optimal code. Yet, a TFCI codeword output from the structure shown in FIG. 5A has a minimum distance of 8 because an error in at least one of word 1 and word 2 during decoding results in an error in the whole 10 TFCI bits. That is, although extended TFCI bits are encoded separately in halves, a minimum distance between final TFCI codewords is equal to a minimum distance 8 between codeword outputs of the biorthogonal encoders 502 and 504.
Therefore, a TFCI codeword transmitted from the encoding structure shown in FIG. 5A is not optimal, which may increase an error probability of TFCI bits in the same radio channel environment. With the increase of the TFCI bit error probability, the receiver misjudges the data rate of received data frames and decodes the data frames with an increased error rate, thereby decreasing the efficiency of the IMT 2000 system.
According to the conventional technology, separate hardware structures are required to support the basic TFCI and the extended TFCI. As a result, constraints are imposed on implementation of an IMT 2000 system in terms of cost and system size.